1. Field of the Invention
This relates to an information recording medium and a process for producing the same. More particularly, it relates to an information recording medium wherein a disc-shaped resin film is joined to a disc-shaped substrate and a process for producing the same.
2. Description of Prior Art
Information recording medium using high-energy-density beams such as laser beam have been developed and put to practical use in recent years. The information recording medium is called optical disc and can be used in the fields of video disc, audio disc and disc memory for large-capacity static image files and large-capacity computers. The optical disc which is put to practical use can be classified into a reproduction type and a Direct-Read-After-Write (DRAW) type.
Conventional DRAW type optical disc basically comprises a recording layer provided on a disc-shaped transparent substrate of a plastic material, glass, etc. An undercoat layer or intermediate layer composed of a polymer is sometimes provided on the surface (on which the recording layer is provided) of the substrate from the viewpoints of improving the smoothness of the substrate, the adhesion of the substrate to the recording layer and the sensitivity of the optical disc. Further, it is necessary to protect the recording layer from external influences, because it is susceptible to external influences. The writing of information on the recording layer is done, for example, by irradiating a recording medium with laser beam. Recording is done by the physical or chemical change of the irradiated portions of the recording layer. As a method for recording information on the recording layer, there is known a heat mode recording wherein the recording layer is irradiated with laser beam to evaporate a recording material, thus providing pits. Japanese Patent Publication No. 59(1984)-5115 discloses that a disc-shaped resin film is joined to one side (on which the recording layer is provided) of the substrate in an information recording medium, wherein the recording layer is protected, for recording information by the heat mode recording. In the heat mode recording, it is desirable to provide a space for oxygen required for the combustion of a recording material or for diffusion of the evaporated gas of the recording material in the formation of the pits. In the above Patent Publication, the resin film is in contact with the recording layer provided on the substrate, so that there is provided no space. Thus, the recording medium described in the above Patent Publication is unsuitable for use in forming pits by the irradiation of low-energy laser beam.
In order to solve the above problems, there has been proposed a technique in which a transparent protective film having protruded and recessed parts formed on one side thereof is provided on the recording layer side of the substrate in such a manner that the protruded and recessed surface is brought into contact with the recording layer to thereby form a space between the recording layer and the film (see, Japanese Patent Provisional Publication No. 59(1984)-36339). The protruded part of the transparent film is in contact with the recording layer. However, the recessed part thereof is not in contact with recording layer. Thus, when the recording layer is irradiated with laser beam to form the pits, there is a difference in the combustion and evaporation of the recording material between the part where a space is formed by the recording layer and the recess, and the part where the recording layer is in contact with the protruded part. Accordingly, difference and unevenness in recording characteristics therebetween are likely caused.